The use of NMR tools for well logging has proved to be a valuable means of analyzing formation characteristics. One disadvantage of current NMR logging apparatuses is their slow logging speeds. Logging speeds for these instruments are typically in the range of between 200 to 300 ft./hr. One of the main reasons for the slow measurement speed of these instruments, is the inherent wait time between their successive measurements. The antenna receiving the signals must, by necessity, wait for the induced magnetization to reach proper spin levels before the next measurement can be taken. Wait times of as much as eight seconds have been necessary in vuggy carbonate formations, owing to the long T.sub.1 relaxation time of the fluids, such as: methane, water, and light oils that are contained in these formations.
Recently, a new NMR logging technique has been able to acquire bound fluid porosity measurements at much faster logging speeds. (See, for example. J. M. Singer, L. Johnson, R. L. Kleinberg, and C. Flaum. Fast NMR Logging for bound Fluid and Permeability Logging, presented at the SPWLA Annual Logging Symposium, 1997.) For free fluid porosity measurements, however, this technique requires an additional nuclear log.
The present invention features a new NMR logging tool and method that can be used to measure both bound fluid, and free fluid porosity with a logging speed, which has been successfully tested at up to 4200 ft./hr. The wait times between successive measurements of the new NMR logging technique are in the order of about one second. This shortened wait time is even applicable for formations displaying relaxation times of several seconds.
The invention comprises an NMR instrument that has a long prepolarization region of up to about six feet, and a short antenna length of approximately six inches. This unique tool configuration allows the regions ahead of the antenna to sufficiently polarize despite the higher tool speed, such that measurements can be taken in quick succession.
For a given logging speed, v, the wait time T.sub.w between measurements is chosen in such a way that the sensitive regions of two subsequent measurements are not overlapping. Since the sensitive region along the tool direction is typically of the order of the length of the antenna, L.sub.antenna, or slightly longer, the logging tool should move about an antenna length during the wait time T.sub.w. Therefore, the wait time should be approximately T.sub.w =L.sub.antenna /v. This ensures that the NMR measurement is not affected by the previous measurement.
For logging tools that can be operated at different rf frequencies (see for example patent by Strikman et al., U.S. Pat. No. 4,710,713), measurements at different rf frequencies can be interleaved. As long as the rf frequencies differ by more than the nutation frequency .gamma.B.sub.1, the sensitive zones for the different rf frequencies are not overlapping. Here .gamma. is the gyromagnetic ratio and B.sub.1 is the strength of the rf field in the formation. For any given rf frequency, the wait time between measurements should be T.sub.w =L.sub.antenna /v, but measurements at different rf frequencies can be performed during this time without causing any interference.
As long as the measurements are non-overlapping, the initial amplitude of the CPMG sequence does not depend on the wait time. Note that this is true even if the longitudinal relaxation time, T.sub.1, is longer than the wait time T.sub.w.
The length of the prepolarization region is preferably long compared to vT.sub.1,max, where T.sub.1,max is the longest relaxation time encountered in the formation. Such a prepolarization region assures that all the spins are exposed to the magnetic field of the prepolarizer for a sufficiently long time. This ensures that they are fully polarized by the time they enter the sensitive region where the NMR measurement is performed. Combined with the wait time discussed above, the measured initial NMR echo amplitude is then directly proportional to the porosity times the hydrogen index. With this scheme, fast logging speed is combined with high spatial resolution. At high logging speed, this spatial resolution, given by the antenna length, can be much higher than vT.sub.1.
If the length of the prepolarization region is shorter than vT.sub.1,max at the highest desired logging speeds (e.g. for practical limitations on length or weight of the tool), a correction will have to be applied to the measurements to obtain the correct porosity for formations with long T.sub.1 relaxation times. This correction will depend on T.sub.1, the logging speed, and the length and field profile of the prepolarization region. The present disclosure minimizes this correction. The longer the prepolarization region, the smaller the correction. For overlapping measurements (i.e. shorter wait times than discussed above), the correction becomes larger and more complicated, as it now depends also on the wait time used.
If the corrections are significant, the longitudinal relaxation times T.sub.1 have to be determined by a combination of non-overlapping measurements and one or several overlapping measurements. This corresponds to a generalized form of multi-wait logging (see for instance U.S. Pat. No. 5,486,762).
One implementation of this configuration would be for an excentric tool, which is applied against the borehole wall. As aforementioned, this tool design would make it possible to acquire NMR porosity at up to 4200 ft./hr. Such a tool would have a maximal resolution of about one foot. This configuration would not affect any of the present NMR applications.